High speed printing machines have been developed in recent years for printing data on computer paper, business forms and the like. The paper supplied to such printers is provided in webs of indeterminate length formed with longitudinally spaced, transversely oriented perforations. The paper is fed at high speeds from the web to the printer which prints the desired information on the individual sheets formed between adjacent perforations, and then discharges the printed sheets for further handling.
In order to convert the continuous length of paper from the printer into a form which can be handled and shipped, the paper first must be folded along its perforations after it is discharged from the printer. One type of folding apparatus intended for use with high speed printers is a spiral, zigzag folder of the type disclosed in U.S. Pat. No. 4,828,540, which is owned by the assignee of this invention. Spiral zigzag folders crease the continuous length of paper at the longitudinally spaced perforations to form folded sheets which are interconnected at the perforations. Because the webs which supply paper to the printers and zigzag folders are of indeterminate length, an essentially continuous stack of printed, zigzag folded sheets is discharged from the zigzag folder in the course of a production run. A separating device therefore must be provided to cut or separate the continuous main stack of folded sheets at selected intervals to form smaller, separated stacks of folded sheets which can be boxed for handling and shipment.
A number of stack separating devices have been developed for this purpose, and representative devices are disclosed, for example, in U.S. Pat. Nos. 4,618,340; 4,396,336; 4,778,165; 4,718,654; 4,750,724; and, 4,508,527. Stack separating devices of the type disclosed in these patents are either associated with a particular folding machine, or are separate and located downstream from the folder. In either type of separating device, the folded sheets are discharged from the folder onto a horizontally oriented support plate which is movable vertically downwardly as successive folded sheets are discharged from the zigzag folder. At selected intervals, when a sufficient number of sheets have accumulated on the horizontal support plate, a separator plate is extended between two adjacent sheets of the stack so that a downstream portion of the stack is formed which is located between the separator plate and the horizontal support plate, and a main, upstream stack is formed which is located between the separator plate and the folder. A cutter mechanism associated with the separator plate, or, alternatively a separate cutter mechanism, is then actuated to break the perforation between the two adjacent sheets spread apart by the separator plate. After separation, the downstream, separated stack is moved by the horizontal support to a take away location during which time the separator plate remains in place to support the main stack of folded sheets accumulating between the separator plate and folder. After the horizontal support plate has removed the separated stack of folded sheets, it returns to an initial position beneath the main stack of folded sheets thus allowing the separator plate to be withdrawn in preparation for another separation operation.
One problem with separating devices of the type described above is that separation of two adjacent sheets within a growing, vertical stack of folded sheets can be a difficult operation. If the folder is operated at relatively high speed, the stack of folded sheets grows rapidly in height and weight. The separator plate must be advanced in a horizontal direction between two adjacent sheets of this vertically growing stack without displacing the stack in a horizontal direction as it moves into the stack. Additionally, the separator plate must overcome the weight of the folded sheets located above it without ripping or tearing the sheets upon entering the stack. Each of these problems can produce ripping or tearing of the folded sheets and/or displacement of the vertically oriented stack in the course of a cutting operation. These problems can result in requiring operation of the folder at slower speeds, so that the rate of formation of the vertically oriented stack is reduced.
Another type of stack separating device which has been proposed as an alternative to that described above is disclosed, for example, in U.S. Pat. Nos. 4,747,591 and 4,507,109. In these stack separating devices, the folded sheets are discharged from a zigzag or other type of folder onto a conveyor which transmits the sheets on their folded ends, e.g., horizontally, instead of in a vertically oriented stack. A cutter mechanism is provided downstream from the folder which separates two adjacent sheets within the horizontally oriented stack, cuts or breaks the perforation between such sheets and then conveys the separated stack of sheets to a location for further handling. While devices of this type solve some of the problems associated with the separation of vertically oriented stacks, other problems are nevertheless created. For example, neither of the devices disclosed in U.S. Pat. Nos. 4,747,591 and 4,507,109 provide for adequate displacement of the adjacent sheets to be separated so that the cutting or separating operation can proceed without tearing of either sheet.